[Dual Shade] Expand the other shade when swiping down the top edge.

In ag/34710720 we ensured the shade invocation zones are the entire
empty half of the screen (when a shade is already open), but in our
gesture detection that area does not overlap with the top edge of the
screen, which is detected separately. Therefore, we re-add the same
user action mapping for the corresponding top edge sides.

Fix: 434662377
Fix: 433545749
Test: Tested manually by swiping down on the empty side's top edge when
 either shade was open (on a wide-screen device), and verifying the
 expected behavior.
Test: Updated unit tests.
Flag: com.android.systemui.scene_container
Change-Id: I4890b8727c8370de62cc7cfa5b200865e9fc7cc6